dBi stands for deciBel Isotropic. It is the gain of an antenna relative to that of an ideal isotropic antenna that radiates energy equally in all directions.
dBm is the power relative to one milliwatt. dBm can be directly converted to Watt and vice versa.
Use this calculator to find the maximum output power of a transmitter in dBm. It uses:
- Antenna gain (dBi)
- Cable Loss (dB)
- Effective Isotropic Radiated Power (EIRP)
dBm to dBi Formula
PTx = EIRP + LC – GAnt
Example Application
The maximum EIRP for a LoRa device operating at 922 MHz in Japan is +27 dBm.
Let’s say the antenna gain is specified to be 2 dBi. Using these numbers and a cable loss of 1 dB, the maximum output power from the transmitter is restricted to +26 dBm.
In the event that the antenna gain is specified to be 10 dBi, the max output power is limited to only +18 dBm.
As the antenna gain increases, the amount of radiated power in a particular direction increases. The antenna beam becomes more focused. For a fixed EIRP this translates to a requirement to reduce the transmitter power to keep within FCC limits.
Background
An antenna’s dBi value is a measure of its gain compared to a an isotropic antenna. Isotropic antennas radiate signal equally in all directions. An antenna with a higher dBi value has more gain than an antenna with a lower dBi value.
Antenna gain can be calculated from Antenna Factor and Frequency.
When choosing an antenna for a particular application, it is important to consider its gain.
A high gain directional antenna can be a good choice for applications where there is a requirement to maximize the signal strength in a specific direction. This is the case when transmitting a radio signal at a specific target. It’s also the case when receiving a signal from a specific direction.
Directional antennas can also make it more difficult to receive signals from other directions, so they are not always the best choice for every application.
Antennas for cell phone boosters and TVs are often directional to maximize signal from a fixed cell tower or TV station. In this post we discussed a higher gain (8 dBi) antenna for Helium systems.